The use of earth leakage circuit breakers (ELCBs) is well known to protect against earth leakage in an appliance having an electrically conductive housing which is connected to ground. In such case, if a fault occurs in the appliance whereby the casing becomes live, then the earth leakage current flows to ground and immediately operates the ELCB. However, if the ground connection is faulty such that there is no leakage path for current to flow in the event that the casing becomes live, then the ELCB will not become energized until somebody touches the electrically conductive housing and thereby provides a leakage path to ground. In such case, the leakage current passes through the person giving rise to the required imbalance between the live and neutral feeder currents which causes the ELCB to operate. Under these circumstances, whilst the ELCB will still operate, there is an inevitable earth leakage through the person who touches the appliance.
In order to address this drawback, published PCT application No. WO 95/31028, in the name of the present applicants, discloses a detector for monitoring the integrity of a ground connection to an electrical appliance having live and neutral terminals for feeding current to the appliance from respective live and neutral feeders of an electrical supply having a ground point for connecting to the ground terminal of the appliance. The detector comprises a differential comparator circuit for comparing a voltage at the neutral connection with a voltage at the ground terminal of the appliance and producing a fault signal if a difference therebetween exceeds a predetermined threshold. A switching device is connected in at least one of the live and neutral connections so as to be opened by a relay operatively coupled to the detector and responsive to the fault signal produced thereby.
The detector disclosed in WO 95/31028 is much more reliable than hitherto proposed ELCBs to protect against electric shock in the event of a poor ground connection. However, for the appliance to operate, a functional ground connection is still required. Thus, if there is no ground connection or its impedance is too high, then the detector disclosed in WO 95/31028 trips the supply to the appliance in order to prevent the risk of electric shock.
This means that if the ground connection to an electrical socket is impaired, then no electric power is available from the socket until a good ground protection is restored. Such repair is obviously only within the capabilities of skilled personnel and thus results in delays and inconvenience to the consumer whilst the socket is unusable. Furthermore, the detector disclosed in WO 95/31028 is associated with an electrical appliance and not with an electrical socket outlet, thus effectively increasing the cost of the appliance.
Yet a further consideration relates not so much to the integrity of the ground connection, but rather to the insulation between the ground and live connections. In an installation conforming to accepted standards, the impedance between the live and ground feeders is in excess of 1.5 M.OMEGA.. However, old installations having many appliances connected thereto are known wherein the impedance between the live and ground feeders can fall to such a low level that the ground feeder, far from protecting a user, can actually conduct fatal leakage current to the metal casing of an appliance.
One known approach to the problems outlined above has been to employ ELCBs but to do away altogether with the ground connection. To this end, so-called "double insulated" appliances are provided wherein all the electrical connections to the appliance are shielded from the outer casing by means of an inner, electrically insulating casing, which itself is spaced apart from the outer casing usually by means of electrically insulating spacers. There is then no need for a ground connection and only live and neutral connections, via a standard two-core cable, are provided.
Although this solution is effective, it increases the cost of the appliance. Moreover, typically the electrically insulating material employed in such appliances is thermosetting plastics which are environmentally unfriendly.
It would clearly be desirable to enjoy the advantages of the detector disclosed in WO 95/31028 whilst, at the same time, not incapacitating an electrical socket outlet whose ground connection is impaired and preferably not requiring integration within an electrical appliance.
EP 695 105 discloses a protection device for use in conjunction with an electrical appliance so as to protect against the casing thereof becoming "live". The electrical appliance has a ground connection 33, which might be the metal casing of the appliance, which is connected via a resistor R1 to a protection circuit comprising elements EC2, EC3, EC4 and OC1 so that if the leakage current flowing through R1 exceeds a certain threshold, this protection circuit provides a trigger to a relay coil RL1 so as to open the primary switch contacts SW1 and SW2. Thus, the protection circuit produces a fault signal if the ground impedance falls below a predetermined threshold.
Likewise, FR 2468430 discloses a protection device wherein, as in above-described EP 695 105, the principle of operation is that, in the event of a ground fault, there will be a leakage current flowing through the ground connection and the magnitude of this leakage current is employed in order to provide a trip signal for the main circuit breaker(s). It is true that in the case of a regular ground fault between either the live or neutral feeders and GND, the resulting ground leakage current which flows through the virtual ground connection effects adequate protection. Therefore, if all that is of concern is ground leakage owing to what is normally referred to as a phase-fault, then the circuits of EP 695 105 and FR 2468430 provide protection of acceptable scope.
However, in the event of a short-circuit between the live and neutral connections, there will under normal circumstances be no ground leakage current and therefore the circuits described in EP 695 105 and FR 2468430 will afford no protection. This is a very serious drawback because short-circuit faults represent a significant risk of fire.